This invention relates to an apparatus for separation or concentration of liquids, separating mixtures of gasses and ultrafiltration of liquids. In particular it pertains to a spirally wound element of uniquely configured layers which support a thinly deposited differentially permeable membrane capable of separating various liquids or gases.
Spirally wrapped semi-permeable membrane fluid separation apertures are well known. Examples are described in U.S. Pat. Nos. 3,367,504, 3,417,870, 3,872,014 and 3,962,096. The sprially wrapped module characteristically employs one or more leaves with each leaf made up of two permselective membranes spaced from each other by a separator grid material through which fluid feed flows. Typically, the membrane is formed from a single elongated membrane sheet, which is folded intermediate of its length to provide a membrane sandwich with the porous material between the membrane folds and with the active separation surfaces of the opposing folds of the membrane sheet away from the product spacer, thus forming a sandwich structure. A grid material is positioned immediately adjacent thereto and upon spirally wrapping the membrane sandwich and backing material tightly about a central mandrel, there is formed a compact roll which is encased in a cylinder, close fitting pressure vessel. The fluid (brackish water, whey or gas mixture) being processed is admitted to one end of the pressure vessel and flows through the separate grid between the adjoining folded membrane leaves of the sandwich in a direction generally parallel to the central mandrel exiting the opposite end. In a typical fluid separation, such as brackish water, a substantially purified product water (permeate) diffuses through the opposing membrane leaves of the sandwich and is absorbed into the porous backing material which transports the product water in a generally radial direction through perforations to the interior of the mandrel through which the purified water product is removed to the exterior of module.
In the field of treatment of water and other liquids it is well known that the channels provided to carry the brine feed become clogged with unfiltered solids or in the case of slurries, a destructive pressure drop must be maintained across and element to force thick viscous material through the element where the membrane extracts liquid. U.S. Pat. No. 3,962,096 pertains to this problem but does not deal with the unavoidable diminishing velocity through the brine channel as permeate or product is extracted. This flow velocity component is necessary for both liquids and gases in order to avoid concentration polarization at the membrane which retards or blocks permeation through the membrane. The prior art considers a tortuous path to be the preferred routing for brine flow to reduce the potential for concentration polarization. There is no data available to confirm this assumption.
Channels of constant cross-section are utilized in prior art construction of product separator. A cross-grid mesh with channels running helixally along the element axis constitutes prior art construction of the brine spacer. These designs pose problems for both the brine channels and the product flow space. The latter carries permeate from the membrane inner surface to the product water tube. As aforementioned, the brine velocity diminishes through the element as permeate is extracted. This reduced velocity increases the concentration polarization and, thereby, reduces the permeation through the membrane at the exit end of the element.
Channels carrying the permeate to the product tube are of the same cross-section along the leaf. No allowance is made for the accumulation of permeate near the product tube, thus, a restriction is imposed reducing the permeate flow.
The product tube is the center of the spirally wound element and the collecting manifold for the permeate. No special allowance is made on current devices for the free flow distribution of the permeate through admitting holes into the product flow channel. The present devices include a plurality of small holes that are partially blocked initially by the first layer of membrane backing material which is generally material known by the trademark TRICOT or SIMPLEX that enwraps the product tube.
In the spiral winding of the elements, the different radius that is assumed by each successive layer requires sliding or relative movement therebetween. This relative movement often damages the membrane layer which must slide relative to the brine space material. The brine space material is generally constructed of highly porous extruded polyurethane or polypropylene. Sharp edges or flashings often damage the membrane surface as the rolling is being done.
Much of the state of the art spirally wound element technology was developed when the flux through membranes was very low and before the introduction of certain improved membrane films which made the process of ultrafilteration and gas separation attractive. Therefore, the state of the art configurations restrict both brine flow and permeate flow for all applications and particularly for the high flux applications of ultrafilteration and gas separation.
Element leaves that are too long have by prior art been shown to have a reduced flux when compared with bench tested membrane samples. This has led to multiple shorter leaf element designs. A limit still exists at the product water tube since the product converges to the product tube at the center of the element. The diameter of larger elements (6" or so) is limited both by pressure consideration on the containing vessel (but not so with low pressure high flow systems) and by the flux reduction at the extreme tips of the leaves.
The present invention provides a spirally wound multi-element module for separating a permeate from a fluid feed mixture which retains the advantages, but eliminates the above referenced defects of the prior art.